1. Field of the Invention
The present invention relates to a process for producing a molded article having a void inside, which molded article is made of a thermoplastic resin. The thermoplastic resin is, hereinafter, referred to as a "resin". The molded article having the void inside and made of the resin is referred to as a "resin hollow molded article" or a "hollow molded article".
2. Description of Related Art
Japanese unexamined patent publications Nos. hei 4-212822 and hei 7-144336 disclose a process for producing a hollow molded article utilizing a core retreating method. In this process, a molten resin is injected with an extremely high pressure into a cavity which is completely closed with a predetermined clearance. After filling the cavity with the resin, a movable core is retreated to expand the cavity while injecting the high pressure gas into the molten resin. Consequently, a void is formed in a resin substrate. However, in such a hollow molding process utilizing an injection molding method, an extremely high clamping force must be applied to a mold because the molten resin is injected with a high pressure in order to fill the cavity. Therefore, a clamping machine inevitably becomes large and expensive. Additionally, a molded article to be obtained suffers from deformation and warp due to residual stress. Moreover, a pressure to be applied to the movable core is very high because the molten resin is injected into the cavity with a high pressure. Hence a machine for holding the movable core and retreating the same is obliged to become large in order to hold the movable core at a predetermined position with opposing to the high pressure of the molten resin and in order to retreat it smoothly.